1. Technical Field
The invention relates generally to stop-motion mechanisms, and more particularly a stop-motion mechanism incorporating a threaded rotatable shaft and a follower for engaging the thread on the shaft.
2. Discussion
Visual projectors of various types project an image imprinted upon a transparent film onto a viewing screen, such as microfiche readers and reel-to-reel microfilm readers. The film, whether microfiche or microfilm, is generally arranged having a series of adjacent frames, each frame having the same rectangular size. A microfiche film generally has a two-dimensional array of frames in an adjacent grid format. A reel of microfilm generally holds a long strip of having only one frame across its width, and the frames are arranged end to end. Microfiche films are generally imprinted with a series of images which are arranged in rectangular frames in a two-dimensional array, in which each frame is of the same size and is aligned adjacent to the surrounding frames. Each frame on either type of film is usually imprinted with one of a series of consecutive images, such as pages from a book, periodical, newspaper, catalog, or patent. In addition, films are often provided with indexing holes arranged in preselected positions for being engaged by indexing projections, posts, or gears provided on the projector, so that the film will always be loaded in the same position relative to the projection apparatus. As a result, the film is preferably aligned so that each frame may be located by an operator of the visual projector, and that each frame will be aligned with respect to the viewing screen.
The visual projector, whether of microfiche or microfilm, is generally adapted to show one frame at a time. Each projector is generally arranged to have a constant magnification, except that some projectors allow the lens to be removed and replaced. In other words, the projector cannot zoom in on a particular area or view more than one frame at a time, without physically changing the lens.
Such visual projectors utilize various mechanisms for moving the transparent film relative to the projection apparatus, so that the desired portion of the film will be projected onto the screen. Microfiche readers generally provide a film carrier constructed of a transparent and clear platen and cover. A rectangular sheet of film is placed between the platen and the cover, and the carrier can be manually moved by manipulating an arm which is rigidly affixed to the carrier. The carrier is generally free to move horizontally in two perpendicular dimensions with respect to a projection lens, so that any particular portion of the film may be viewed.
In contrast, microfilm projectors utilize a reel-to-reel driving system, in which two reels are driven by motors so that the operator may move the film forward or backward, while the projection apparatus displays a portion of the film having the same size as one of the frames imprinted thereon.
The various mechanisms for moving the transparent film in a visual projector are thus generally of two types: manual and motor-driven. In a manual mechanism, the operator merely moves by hand a pointer which is rigidly affixed to a carrier which holds the film. In a motor-driven mechanism, such as in a reel-to-reel microfilm projector, the operator can run the film either forward or backwards, at a desired speed among a range thereof. In general, existing methods for moving the transparent film used in visual projectors are generally capable of moving that film only in a generally continuous manner. As a result, the images and frames shown on the viewing screen generally move rapidly past the projecting apparatus as the film is moved to shown another portion thereof, presenting an incomprehensible blur on the viewing screen.
Accordingly, it is desirable to provide a mechanism for moving a transparent film for use in a visual projector in an intermittent stop-motion fashion, whereby the film will pause momentarily to show an individual frame, move the film to another frame, and then pause to project that frame momentarily. The frames would appear to be stationary while the images would appear to be animated, because of the optical effect of image holding, rather than simply moving or whizzing past on the screen, with the frames appearing to blur in motion.